Polyurethane elastomers

ABSTRACT

The present invention is directed to a solid polyurethane composition having a glass transition temperature of from -20° C. to 0° C., having a Bayshore rebound of less than 20, and having a Shore A hardness of less than 80.

BACKGROUND OF THE INVENTION

Low resiliency elastomers with good impact and shock absorption are potential candidates for shoe insoles, gloves, damping pads, vibration dampers, shock absorber pads or any other application where a soft elastomer with these properties is required. The use of polyurethane elastomers as such energy absorbing media is known. See, e.g., U.S. Pat. Nos. 4,346,205, 4,476,258, 4,485,719, 4,552,944, 4,567,088, 4,604,940 and 4,722,946. Additionally, elastomers prepared from isocyanate terminated prepolymers and relatively low molecular weight hydroxyl group containing compounds are also known. See, e.g., U.S. Pat. No. 4,551,498.

While the prior art compositions have properties adequate for some applications, it has heretofore not been possible to produce an elastomer from a two component polyurethane system where the two components could be reacted in a weight ratio of from 1.5:1 to 1:1.5, and which system would yield elastomers with relatively low hardnesses and low rebounds, and with glass transition temperatures of from -20° C. to 0° C.

DESCRIPTION OF THE INVENTION

The present invention is directed to an elastomer which satisfies the above criteria. In addition, the systems of the present invention can generally be processed and demolded in relatively short cycle times. Finally, the elastomers of the present invention have overall good physical properties and specifically have good tear strength. More particularly, the present invention is directed to a solid polyurethane composition having a glass transition temperature of from -20° C. to 0° C., having a Bayshore rebound of less than 20, and having a Shore A hardness of less than 80, and being prepared by reacting:

a) a liquid, urethane group containing, isocyanate, prepared by reacting methylenebis(phenyl isocyanate) with a diol having a molecular weight of less than 450, and having an isocyanate group of from about 15 to about 25% by weight, with

b) a hydroxyl functional compound selected from the group consisting of

i) one or more diols having molecular weights of from 62 to about 1000,

ii) one or more triols having molecular weights of from about 500 to about 2000, and

iii) a mixture of

1) one or more triols having molecular weights of from about 500 to about 3000, and

2) one or more diols having molecular weights of from about 200 to about 3000, with the weight ratio component iii)1) to component iii)2) being from 2:1 to 1:1, and,

c) no more than 10% by weight based on the weight of component b) of one or more triols having molecular weights of less than 500,

and wherein the isocyanate to hydroxyl equivalent ratio is from 0.95:1 to 1.2:1, and the weight ratio of component a) to component b) being from 1.5:1 to 1:1.5.

All of the various reactants are known in the art. The liquid, urethane group containing isocyanates useful as starting components for the production of the elastomers of the present invention are known and are described in U.S. Pat. No. 3,644,457, the disclosure of which is herein incorporated by reference.

The starting components used for the production of the elastomers of the present invention also include a variety of different hydroxyl group containing compounds which contain two or three hydroxyl groups.

The hydroxyl component to be reacted with the isocyanate component to form the elastomer of the present invention is a hydroxyl functional compound selected from the group consisting of

i) one or more diols having molecular weights of from 62 to about 1000,

ii) one or more triols having molecular weights of from about 500 to about 2000, preferably polyether triols having molecular weights of from about 500 to about 2000, and

iii) a mixture of

1) one or more triols having molecular weights of from about 500 to about 3000, preferably polyether triols having molecular weights of from about 500 to about 3000,

2) one or more diols having molecular weights of from about 200 to about 3000, preferably polyether diols having molecular weights of from about 300 to about 3000, with the weight ratio component iii)1) to component

iii)2) being from 2:1 to 1:1.

In addition, as noted above, up to 10% by weight of a relatively low molecular weight triol can also be used.

The hydroxyl compounds useful in producing the elastomer are generally known in the polyurethane art. All that is necessary is that the molecular weights and amounts of materials be as noted above. Such compounds include, for example, polyesters, polyethers, polythioethers, polyacetals, polycarbonates and polyester amides containing at least 2, and no more than 3 hydroxyl groups, such as those known for the production of polyurethanes. A key to the present invention resides in the proper selection of hydroxyl compound for production of the elastomer.

Hydroxyl group-containing polyesters used may be, for example, reaction products of polyhydric, preferably dihydric alcohols, optionally with the addition of trihydric alcohols, and polybasic, preferably dibasic carboxylic acids. Instead of using the free polycarboxylic acids, the corresponding polycarboxylic acid anhydrides or corresponding polycarboxylic acid esters of lower alcohols or mixtures thereof may be used for preparing the polyesters. The polycarboxylic acids may be aliphatic, cycloaliphatic, aromatic and/or heterocyclic and they may be substituted, e.g. with halogen atoms, and/or be unsaturated. The following are mentioned as examples: succinic acid, adipic acid, suberic acid, azelaic acid, sebasic acid, phthalic acid, isophthalic acid, trimellitic acid, phthalic acid anhydride, tetrahydrophthalic acid anhydride, hexahydrophthalic acid anhydride, tetrachlorophthalic acid anhydride, endomethylene tetrahydrophthalic acid anhydride, glutaric acid anhydride, maleic acid, maleic acid anhydride, fumaric acid, dimeric and trimeric fatty acids, optionally mixed with monomeric fatty acids, dimethylterephthalate and terephthalic acid-bis-glycol esters. Suitable polyhydric alcohols to produce such polyesters include, e.g. ethylene glycol; propylene glycol-(1,2) and -(1,3); butylene glycol-(1,4) and -(2,3); hexanediol-(1,6); octanediol-(1,8); neopentyl glycol; cyclohexane dimethanol (1,4-bis-hydroxymethylcyclohexane); 2-methyl-1,3-propanediol; glycerol; trimethylolpropane; hexanetriol-(1,2,6); butanetriol-(1,2,4); trimethylolethane; diethylene glycol; triethylene glycol; tetraethylene glycol; polyethylene glycols; dipropylene glycol; polypropylene glycols; dibutylene glycol and polybutylene glycols. Polyesters of lactones, such as ε-caprolactone, and hydroxycarboxylic acids, such as ω-hydroxycaproic acid, may also be used.

The polyethers used according to the present invention contain at least 2 and no more than 3 hydroxyl groups are known. They may be prepared, for example, by the polymerization of epoxides, such as ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, styrene oxide or epichlorohydrin, either each on its own, (e.g. in the presence of BF₃) or by chemical addition of these epoxides, optionally as mixtures or successively, to starting components having reactive hydrogen atoms, such as water, alcohols or amines, e.g. ethylene glycol, propylene glycol-(1,3) or -(1,2), trimethylol propane, 4,4'-dihydroxydiphenylpropane, aniline, ammonia, or ethanolamine. It is frequently preferred to use polyethers which contain predominantly primary OH groups (up to 90%, by weight, based on all the OH groups present in the polyether). Polyethers which have been modified with vinyl polymers, for example the compounds obtained by the polymerization of styrene and acrylonitrile in the presence of polyethers (U.S. Pat. Nos. 3,383,351; 3,304,273; 3,523,093 and 3,110,695 and German Patent 1,152,536) are also suitable. Polybutadienes containing OH groups may also be used.

Among the polythioethers which should be particularly mentioned are the condensation products obtained from thiodiglycol on its own and/or with other glycols, dicarboxylic acids, formaldehyde, aminocarboxylic acids or amino alcohols. The products obtained are polythio mixed ethers, polythio ether esters or polythioether ester amides, depending on the co-components.

Suitable polyacetals include, e.g. the compounds which may be obtained from glycols, such as diethylene glycol, triethylene glycol, 4,4'-dioxethoxy-diphenyl-dimethylmethane or hexanediol, and formaldehyde. Polyacetals suitable for the purpose of the present invention may also be prepared by polymerizing cyclic acetals.

Suitable polycarbonates containing hydroxyl groups are known and may be prepared, for example, by the reaction of diols, such as 1,3-propanediol, 1,4-butanediol and/or 1,6-hexanediol, diethylene glycol, triethylene glycol or tetraethyleneglycol, with diarylcarbonates, e.g. diphenyl carbonate, or phosgene.

Suitable polyester amides and polyamides include, e.g. the predominantly linear condensates obtainable from polybasic saturated and unsaturated carboxylic acids or their anhydrides and polyvalent saturated and unsaturated amino alcohols, diamines, polyamines or mixtures thereof.

Polyhydroxyl compounds already containing urethane or urea groups as well as modified or unmodified natural polyols, such as castor oil, carbohydrates or starch may also be used. Additionally, products of alkylene oxides and phenol/formaldehyde resins or of alkylene oxides and urea/formaldehyde resins are also suitable according to the present invention.

Also useful are hydroxyl compounds having relatively low molecular weights. They contain 2 or 3 hydroxyl groups. The following are mentioned as examples of such compounds: ethylene glycol; 1,2- and 1,3-propylene glycol; 1,4- and 2,3-butylene glycol; 1,5-pentanediol; 1,6-hexanediol; 1,8-octanediol; neopentyl glycol; 1,4-bis-hydroxymethylcyclohexane; 2-methyl-1,3-propanediol; glycerol; trimethylolpropane; 1,2,6-hexanetriol; trimethylolethane; diethylene glycol; triethylene glycol; tetraethylene glycol; polyethylene glycols; polytetramethylene glycols; dipropylene glycol, polypropylene glycols; dibutylene glycol; polybutylene glycols; 4,4'-dihydroxy-diphenylpropane; dihydroxymethylhydroquinone; diethanolamine; triethanolamine; 3-aminopropanol; and the like.

Representatives of the various hydroxyl group containing compounds which may be used according to the present invention have been described, e.g. in High Polymers, Vol. XVI, "Polyurethanes, Chemistry and Technology", by Saunders-Frisch, Interscience Publishers, New York, London, Volume I, 1962, pages 32-42 and pages 44-54 and Volume II, 1964, pages 5-6 and 198-199 and in Kunststoff-Handbuch, Volume VII, Vieweg-Hochtlen, Carl-Hanser-Verlag, Munich, 1966, e.g. on pages 45 to 71. The only essential requirement in selecting the particular hydroxyl compound for the elastomer production is that the molecular weight limitations set forth above be observed. It is only in using these narrow ranges of hydroxyl compounds that the elastomers of the present invention can be obtained. In addition, the amounts of components used are essential to produce the elastomers of the present invention.

Both the prepolymer and the elastomer itself are prepared using techniques generally known in the art. In general, the prepolymer is prepared by reacting the hydroxyl compound with an excess of the diisocyanate. This reaction is preferably carried out at elevated temperatures of from 50° C. to about 150° C. until all the hydroxyl groups of the hydroxyl compound have disappeared. More details as to the production of the liquid isocyanates can be found in U.S. Pat. No. 3,644,457. In producing the elastomers of the invention, this prepolymer is then mixed with component b). The resultant mixture can then be cast into molds maintained at temperatures of from about 50° C. to about 150° C., where the final curing of the product takes place.

The invention is further illustrated by the following examples in which all parts and percentages are by weight, unless otherwise indicated.

EXAMPLES

In the examples which follow, the following materials were used:

MMDI: a liquid isocyanate prepared by reacting MDI with tripropylene glycol (TPG) in a molar ratio of MDI to TPG of 5:1, with the resultant product having an isocyanate group content of about 23% by weight. The product is commercially available as Mondur PF isocyanate from Mobay Corporation.

DIOL A: a polyoxypropylene glycol having a molecular weight of 1000

TRIOL A: a trimethylolpropane/propylene oxide adduct having a molecular weight of 455

TRIOL B: a triethanolamine/propylene oxide adduct having a molecular weight of 1122

The B-side was prepared by physically blending the hydroxyl compounds and degassing the resulting homogeneous mixture.

A known weight of MMDI was kept stirred at 32° C. in a dry metal can. A calculated amount of Side-B, also at 32° C., was added to it. The mixture was stirred for 30 seconds and poured into a mold preheated to 60° C. The cast part was cured in an oven at 60° C. for 20 minutes, demolded, and post cured at ambient temperature in open air for 16 hours, before the Shore A (ASTM D-2240) and Bayshore Rebound (ASTM D-3574-H) were checked. The glass transition temperature of the product of Example 2, as determined by the Differential Scanning Calorimetry, was -10.1° C.

The formulations used and the results obtained were as indicated in the following table, with Example 1 being a comparative example:

    ______________________________________                                                                   Weight                                                              B-Side,    Ratio,                                                                               Hardness                                       Example                                                                               A-Side  pbw        A:B   Shore A                                                                               Rebound                                 ______________________________________                                         1      MMDI    34 DIOL A  1:1   90A    30                                                     66 TRIOL A                                                      2      MMDI    49 DIOL A  1:1   58A     6                                                     51 TRIOL B                                                      ______________________________________                                    

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. 

What is claimed is:
 1. A solid polyurethane composition having a glass transition temperature of from -20° C. to 0° C., having a Bayshore rebound of less than 20, and having a Shore A hardness of less than 80, and being prepared by reacting:a) a liquid, urethane group containing, isocyanate, prepared by reacting methylenebis(phenyl isocyanate) with a diol having a molecular weight of less than 450, and having an isocyanate group of from about 15 to about 25% by weight, with b) a hydroxyl functional compound selected from the group consisting ofi) one or more diols having molecular weights of from 62 to about 1000, ii) one or more triols having molecular weights of from about 500 to about 2000, and iii) a mixture of1) one or more triols having molecular weights of from about 500 to about 3000, and 2) one or more diols having molecular weights of from about 200 to about 3000, with the weight ratio component iii)1) to component iii)2) being from 2:1 to 1:1, and, c) no more than 10% by weight based on the weight of component b) of one or more triols having molecular weights of less than 500,and wherein the isocyanate to hydroxyl equivalent ratio is from 0.95:1 to 1.2:1, and the weight ratio of component a) to component b) being from 1.5:1 to 1:1.5. 